Regulation and putative function of the Drosophila gap gene Krüppel

The Drosophila segmentation gene Krüppel (Kr) is expressed in a broad band of cells that covers about four-segment primordia in the blastoderm embryo. Examination of size and position of the Kr protein domain in various mutant embryos revealed that the establishment of the domain of Kr gene expression is under the control of the maternal effect pattern organizers which act at the poles. The lack of Kr activity causes a gap in the segment pattern of the embryo which is about twice the size of the Kr expression domain and extends posterior to it. This indicates that Kr activity per se is not directly responsible for the establishment of the pattern elements which are deleted in the mutant embryo. Examination of the molecular lesions in four Kr alleles indicated that each of them is a point mutant within the coding sequence of the Kr gene and each mutation results in a different replacement of a single amino acid within the ‘finger domain’ of the Kr protein. Thus, this region of the Kr protein is essential for Kr function. Since this portion of the Kr protein shares structural homology with the DNA-binding domain of several transcription factors, we propose that Kr acts as a transcription factor on subordinate genes that process the spatial cues provided by Kr activity to establish eventually the segments in the central region of the embryo.